It has been found very helpful to drillers to be able to obtain environmental data and data regarding the operation of the drill bit and drill string while drilling is ongoing. However, the problem of obtaining information from the bottom of a drill hole which is often a narrow, curved hole extending a mile or more into the earth, is a formidable one. The mechanical shocks and other stresses created during drilling among other factors make the use of an insulated communication path along the drill string impractical. The physical environment and depth of drill holes also limits the power input for communications which may be established therein. A number of techniques and devices have been proposed for achieving the transfer of data from within the drill hole to the surface, without the use of separate conducting cables. For example, Silverman in U.S. Pat. No. 2,354,887 proposes the use of a toroid which surrounds the drill string. Alternating circuit introduced into the winding of the toroid produces a magnetic flux in the core of the toroid with the earth acting as the return path to complete a current loop between points on the drill string above and below the toroid. A variety of techniques are used at the surface to measure the drill string currents thus produced. One of these techniques includes a second toroid surrounding the drill string at the surface. A portion of the current introduced into the drill string also flows through this second toroid and thereby produces a measurable voltage at the output terminals of the second toroid as will be apparent to those skilled in the art. Silverman, as well as other prior bore hole telemetry systems, are concerned only with the gross aspect of the signal path, and are either silent as to what modulation techniques are to be employed, transmit pure analogue signals or assume conventional binary digital data transmission. Silverman discloses a means for transmitting analogue data either by modulating a single carrier frequency or by frequency modulation. Silverman does not transmit digital data, and suggests frequency modulation as being preferable to amplitude modulation in transmitting data regarding formations in the earth.
A serious problem with the transmission of electromagnetic data along the drill string as suggested by Silverman is that due to the dispersive character of the earth, current does not simply flow in a single loop along the drill string through the toroids and return through the earth. A theoretical paper, "Theory of Transmission of Electromagnetic Wave Along a Metal Rod in a Conductive Medium," by James R. Wait and David A. Hill, Institute of Telecommunications Sciences, Office of Telecommunications, U.S. Department of Commerce, indicates that the one turn toroid model is an over-simplification; that actually the current exists as an electromagnetic wave loosely coupled to the drill string. According to Waite et al, the principal attenuation mechanism is such that the current on the drill string can be approximated by the equation EQU I.sub.(d) =.nu..sub.o (B[.sigma.,f,d]e.sup.-d/.delta.)
where:
I.sub.(d) =the current on the drill string as a function of distance (d) from the toroid; PA1 .nu..sub.o =the initial voltage excited in the drill string by the toroid; PA1 B [.sigma.,f, d]=a general variable term which varies with earth conductivity, .sigma., the frequency, f, and the distance, d; PA1 .delta.=the skin depth formula, conventionally used in computation involving a conductive earth where .delta..congruent.500/.sqroot..sigma.f meters
The terms B[.sigma.,f, d] and e.sup.-d/.delta. are the variable terms in the formula. However, the latter exponential term is so predominant that for many purposes, the terms B[.sigma., f,d] can be either neglected or considered as a constant.
This theory therefore indicates that to an approximation the attenuation of current in the drill string increases exponentially with distance, frequency and the conductivity of the earth. Since with binary digital data transmission the data rate, and hence the information bandwidth, is keyed to the frequency used for transmission, lower signal frequency inherently results in a lower data rate. When conventional binary digital data transmission is applied to bore hole telemetry as in the case of Silverman, the result is that with limited communications power input, and the drilling depths and conductivities of interest to petroleum drilling, the frequency must be so low that insufficient data rate can be achieved.
One proposed solution to this problem is suggested in U.S. Pat. No. 3,793,632 issued to Still. Still suggests increasing the operating depth, and/or the achievable data rate possible with the telemetry system of Silverman by using a plurality of repeaters along the bore hole and communicating between adjacent repeaters with noninterfering modes. However, in order to obtain adequate data rates to meet the needs of petroleum drilling and the like using binary digital data transmission, it is necessary that the path lengths between repeaters become so short that the number of repeaters could make the technique economically unattractive.